The technical field is generally cognizant of window structures in which a window lite is set within a glazing channel and held therein by a rigid glazing bead. The use of such a bead allows the glazing channel to be open and unobstructed on one side. This arrangement permits the window lite to be placed in the channel from its open side without the need to disassemble the frame in which the channel is formed. This facilitates the manufacture of windows and the replacement of broken window lites. Furthermore, it is advantageous in constructing a rigid window frame to be able to lock the frame members together permanently without having to provide for their disassembly for glazing.
In certain applications, it is considered esthetically advantageous to provide a window structure with an openable sash in which substantially no part of the sash extends beyond the exterior surface of the window lite. However, such an arrangement is impossible when the window lite is to be held in the sash by means of a glazing channel and bead. Furthermore, the lack of a sash member extending beyond the exterior surface of the window lite makes difficult the proper sealing of such a sash when it is closed. Conventional peripheral weatherstripping interferes with the esthetic goal of providing a window sash in which a minimum of sash frame is visible from the exterior side, and no sash structure extends exterior to the window light to provide a surface against which a seal may press.
The field is cognizant of the desirability of reducing heat flow between the exterior and interior sides of metallic window structures. It is conventional to include structural heat transfer barriers in metallic window structures to interrupt what might otherwise be a continuous, metallic path by which heat could flow between the interior and exterior parts of the window structure. However, conventional structural heat transfer barriers are relatively bulky. Their incorporation in a window sash in such a way as to prevent the flow of heat between interior and exterior sides of the window lite frustrates the goal of reducing to the greatest extent possible that part of the sash which is visible from its exterior side.